In a report, the International Crisis Group (ICG) said the number of Islamist insurgent attacks had increased by 15-20 percent in 2013 from a year earlier, and a similar trend continued into the first half of 2014.

"The violence remains ferocious because there are so many grudges and grievances left over from a dozen years of war here," said Graeme Smith, the ICG's senior analyst in Kabul.

The Taliban, driven from power by U.S.-led forces in 2001, claimed responsibility for a series of attacks across the country on Monday at the start of their summer offensive.

Two rockets landed just outside heavily guarded Kabul airport and a roadside bomb barely missed a deputy minister, officials said. The U.S.-led coalition said four rockets hit the U.S.-run Bagram Airbase near the capital.

No one was killed in the attacks.

Insurgents also stormed a government building in the eastern province of Nangahar and about 300 militants attacked a number of checkpoints in the capital of the eastern province of Ghazni, officials said. About a dozen people were killed.

The Taliban have pledged to target Afghan and foreign forces as well as political efforts to end the conflict as Afghanistan prepares to return to campaign mode this week ahead of a second round run-off in the presidential election.

The United States has been trying for months to nail down a bilateral security agreement allowing a small American force to stay beyond a year-end deadline for all foreign combat troops to leave. The purpose would be helping in counter-insurgency operations and further training of Afghan forces.

Afghan President Hamid Karzai has refused to sign the deal, but the frontrunners to replace him, Abdullah Abdullah and Ashraf Ghani, say they will push the pact through.

Smith said the cash-strapped Afghan government was unable to fund its 370,000-strong security force, and unless the international community increased financial assistance the violence would only worsen.

Smith said as foreign troops have pulled out, fighting has changed into a mix of ethnic, tribal and political conflicts. "Across Afghanistan there a thousand little wars playing out in so many ways," he said.

The Taliban threatened to derail last month's first round of the election, but a security crackdown by Afghan forces ensured the vote proceeded without any major incidents.

Insurgents are expected to pose a bigger threat during next month's second round of voting as the snow will have melted, giving them more freedom to move from their mountain hideouts.

The ICG report said the Taliban, who ruled Afghanistan with an iron fist characterised by a fundamentalist form of Islam from 1996 to 2001, had shifted their rhetoric from resisting foreign occupation to an emphasis on confronting the "puppets" or "betrayers of Islam" in the government.

(Additional reporting by Hamid Shalizi and Mirwais Harooni in Kabul; and Rafiq Sherzad in Jalalabad; Editing by Mark Heinrich)